The proposed research consists of the isolation and identification of the androgens secreted by mammalian fetal testes which determine male phenotypic development of the Wolffian and Muellerian ducts, and urogenital sinus derivatives. The experimental design consists of the sequential (a) establishment of a bioassay for androgen activity, based on explanted ducts in organ culture and morphologic and metabolic criteria, (b) demonstration of androgenic activity in culture media of cultured fetal testes, and inhibitors of androgen synthesis and action, and in vivo studies in rabbit fetuses and chick embryos, and the role of adrenal androgens, of mesonephroi, and of ductal metabolism in ductal differentiation. (c) isolation and identification of the active androgens. Ancillary experiments will be included. In addition, the non-testicular sources of C19 steroids will be studied systematically. The synthesis of testosterone, androstenedione and dehydroepiandrosterone from labeled progesterone and pregnenolone, or from C14-acetate, by rat blastocyst, rat and guinea-pig embryos and fetus during development will be determined. The developmental stages are chosen in their relationship to sexual differentiation.